1. Field of the Invention
The present invention relates to a process for preparing a biodegradable resin foam, particularly to a process for preparing a biodegradable molded foam, for example, sheet, film and 7rapping materials such as cup trays, cushionings, protective sheet, protective film, etc., which comprises a starch polymer and a vinyl alcohol resin.
2. Description of the Related Art
In recent years, the interest in environmental problems has been increasing. For example, there is an increasing requirement for the development of disposal technique of synthetic polymeric materials such as plastics. Therefore, much attention has been paid to biodegradable plastics which can displace conventional plastics derived from petroleum.
As biodegradable plastics, U.S. Pat. No. 4,133,784 discloses a composition comprising starch and ethylene/acrylic acid copolymer (EAA) and Japanese Patent Kokai No. 31333/1991 discloses a composition comprising ethylene/vinyl alcohol copolymer (EVOH) and a modified starch.
An expandable material comprising a biodegradable plastic is disclosed in Japanese Patent Kokai No. 298525/1990. The material described in the published patent uses water as a blowing agent, but is not satisfactory for use in, for example, packing in view of elastic property and compressive strength.
Japanese Patent Kokai No. 14228/1990 discloses an expandable material comprising water-containing starch and a substantially water-insoluble synthetic thermoplastic polymer.
Japanese Patent Kokai No. 500833/1992 discloses an expanded article of biodegradable plastic comprising starch and EAA and/or EVOH.
These foams of biodegradable plastic are produced by adding a plasticizer such as glycerin and a blowing agent to a starch/EVOH composition, mixing, melting and extruding them. They have a low density of, for example, 0.6 g/cc. However, there are both of open cells and closed cells in them. The cell size is not uniform. Moreover, they have cavities all over their surface produced by the release of the blowing agent, which gives uneven and rough surface.
In addition, since natural starch contains water, it is alphalized and melted by kneading it in an extruder under an elevated pressure. The melt is expanded by keeping is under atmospheric pressure. The foam thus produced considerably lacks water-resistance and strength in use for an industrial cushioning material.